As an electrolyte material contained in an ion exchange membrane, an electrolyte membrane for a polymer electrolyte fuel cell, etc., for example, an ionic polymer is known wherein one pendant group (or a side chain) has multiple ion exchange groups such as sulfonic acid groups (—SO3H+ groups), sulfonimide groups (—SO2N−(H+)— groups), etc. Such an ionic polymer is less susceptible to swelling with water even when its ion exchange capacity is made high, and thus has good dimensional stability, as compared with an ionic polymer wherein one pendant group has one ion exchange group.
As methods for producing an ionic polymer membrane wherein one pendant group has multiple ion exchange groups, for example, the following methods (1) to (4) are known.
Method (1): A method of synthesizing a monomer wherein one pendant group has multiple ion exchange groups, and polymerizing such a monomer with e.g. tetrafluoroethylene (hereinafter referred to as TFE) (see Non-patent Document 1).
Method (2): A method comprising the following steps (X1) to (X3) (see Patent Document 1).
(X1) a step of converting a —SO2F group in a polymer having the —SO2F group, to a —SO2NH2 group.
(X2) a step of reacting the resulting polymer having the —SO2NH2 group, with FSO2(CF2)3SO2F to convert part of the —SO2NH2 group to —SO2N−H+SO2(CF2)3SO2F group, while cross-linking the —SO2NH2 group to each other.                (X3) a step of converting the —SO2N−H+SO2(CF2)3SO2F group to a —SO2N−H+SO2(CF2)3SO3−H+ group.        
Method (3): A method comprising the following steps (Y1) to (Y3) (see Patent Document 1).
(Y1) a step of converting a —SO2F group in a polymer having the —SO2F group, to a —SO2NH2 group.
(Y2) a step of reacting the resulting polymer having the —SO2NH2 group, with FSO2(CF2)3I to convert the —SO2NH2 group to a —SO2N−H+SO2(CF2)3I group.
(Y3) a step of converting the —SO2N−H+SO2(CF2)3I group to a —SO2N−H+SO2(CF2)3SO3−H+ group.
Method (4): A method comprising the following steps (Z1) and (Z2) (see Non-patent Document 2 and Patent Document 2).
(Z1) a step of reacting a polymer having a —SO2NH2 group, with an excess amount of a compound having at least two —SO2F groups exemplified by FSO2(CF2)nSO2F, to convert the —SO2NH2 group to a —SO2N−H+SO2(CF2)nSO2F group.
(Z2) a step of converting the —SO2N−H+SO2(CF2)nSO2F group to a —SO2N−H+SO2(CF2)nSO3H+ group.